The present invention relates to a film flatness keeping device of a camera for properly keeping the flatness of a film at each exposure
A film holder mechanism located in the film aperture portion of an ordinary camera of the type using a silver salt film is called a "tunnel system."
FIG. 10 shows a sectional view of a single-lens reflex camera including its optical axis. FIG. 11 is an enlarged sectional view showing the structure including a film, a film aperture portion, and a pressure plate The height of each of paired film rails 2 provided on the aperture side of a camera body is different from that of each of paired pressure plate rails 3 receiving a pressure plate 4, by normally 0.2 mm for the 35-mm film. The upper and lower ends of the film 21 are held by such gaps (tunnels), and the rear side of the film 21 is supported by the pressure plate 4. Accordingly, it is exposed while keeping the flatness of some degree Careful study of the tunnel system shows that unevenness of more than 80 .mu.m may be created at the start and end portions of a film winding depending on its material Theoretically, any kind of film must have a portion warped and separated from the surface of the pressure plate 4 by .DELTA.L within the film aperture portion (mainly in its central area), as shown in FIG. 11.
Practically, the separation .DELTA.L is treated as a negligible matter, because it is within the depth of focus. However, the film flatness needs to be considered in cases where it is not a negligible factor in connection with ambient conditions, such as temperature and humidity, and material of a film, and where due to the use of a high performance lens photographing is made at a shallow depth of focus.
A large vacuum device or the like is often used for a special camera for the photoengraving. Such a large device, however, cannot be used for general portable cameras.
A film suction stabilizing mechanism for general portable cameras is disclosed in Japanese Utility Model Examined Publication No. Sho. 62-30019. In the mechanism, a mirror drive lever, which is turned in synchronism with a shutter release, drives an air dumper, to reduce a pressure within a cylinder. A suction tube coupled with the cylinder is connected to the rear surface of a pressure plate that is located on the back cover side of a camera. A film is sucked through small through-holes of the pressure plate. The mechanism is complicated because a mirror drive mechanism, specifically the cylinder, installed on the camera body side should be connected to the pressure plate on the back cover side by the suction tube.
A pressure plate device for keeping the film flatness at the time of photographing, which is used for a camera using a roll film, is disclosed in Japanese Utility Model Examined Publication No. Sho. 54-19863. In a mechanism of this device, an air chamber is provided on the rear side of a film pressure plate. The air chamber is hermetically enclosed by an expandable member, and is expanded by means of a motor and a link mechanism to reduce its internal pressure. To hold a film, it is sucked through small through-holes of the pressure plate. Use of the motor and link mechanism makes it difficult to provide satisfactory response speed.